Physiologically active substances such as various hormones, neurotransmitters, etc. regulate the biological function via specific receptor proteins present on cell membranes. Many of these receptor proteins are coupled with a guanine nucleotide-binding protein (hereinafter sometimes simply referred to as G protein) and mediate the intracellular signal transduction via activation of G protein. These receptor proteins possess the common structure containing seven transmembrane domains and are thus collectively referred to as G protein-coupled receptor proteins (GPCR) or seven-transmembrane receptor proteins (7TMR).
G protein-coupled receptor proteins are present on the cell surface of each functional cell and organ in the body, and play important physiological roles as the target of the molecules that regulate the functions of these cells and organs, e.g., hormones, neurotransmitters, physiologically active substances and the like. Receptor proteins transmit signals to cells via binding with physiologically active substances, and the signals induce various reactions such as activation and inhibition of the cells.
To clarify the relationship between substances that regulate complex biological functions in various cells and organs in the body, and their specific receptor proteins, in particular, G protein-coupled receptor proteins would elucidate the functional mechanisms in various cells and organs in the body to provide a very important means for development of drugs closely associated with these functions.
For example, in various organs, their physiological functions are controlled in vivo through regulation by many hormones, hormone-like substances, neurotransmitters or physiologically active substances. In particular, physiologically active substances are found in numerous sites of the body and regulate the physiological functions through their corresponding receptor proteins. Many unknown hormones, neurotransmitters or many other physiologically active substances still exist in the body and, as to their receptor proteins, many of these proteins have not yet been reported. In addition, it is still unknown if there are subtypes of known receptor proteins.
It is a very important means for development of drugs to clarify the relationship between substances that regulate elaborated functions in vivo and their specific receptor proteins. Furthermore, for efficient screening of agonists and antagonists to receptor proteins in development of drugs, it is required to clarify functional mechanisms of genes for these receptor proteins expressed in vivo and express the genes in an appropriate expression system.
In these receptor proteins, receptor proteins including agonists and antagonists, in which the corresponding ligands are yet unidentified, are referred to as orphan receptor proteins. TGR25 is known as one of the orphan G protein-coupled receptor proteins (WO 02/46394). This protein is also called GPR97 (FEBS Letters 531, 407-414 (2002)) and as its mouse homolog Pb99 is known (Mol. Cell. Biol. 20, 4405-4410 (2000)). Pb99 is expressed in pre-B cells and thymocytes and not in mature B and T cells, implying that it may function in lymphoid development. It is also reported that TGR25 is a receptor expressed in bone marrow (JPA 2003-24070).
It is reported and disclosed that AM251 [1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-piperidin-1-yl-1H-pyrazole-3-carboxamide] is an antagonist of the cannabinoid receptor and has anorectic and weight loss effects [Eur. J. Pharmacol. 462 (1-3): 125-132 (2003)], and has an agonist action on GPR55, one of G protein-coupled receptor proteins (WO 01/86305). It is reported that AM251, AM281 [1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-morpholin-4-yl-1H-pyrazole-3-carboxamide], etc. are cannabinoid receptor CB1-specific antagonists (Life Sci. 76 (12): 1307-1324 (2005)). It is also reported that N-pentanoyl 2-benzyltryptamine (DH97) is melatonin receptor MT2 antagonist [Naunyn Schmiedebergs Arch Pharmacol. 358 (5): 522-528 (1998)].